The invention relates to a method for controlling the braking process in a motor vehicle and a brake system which operates according to such a method. In such a method, brake actuation signals are converted into wheel brake pressures, the wheel speeds of at least some of the wheels are transmitted to a control system, the control system calculates a wheel slip from the wheel speeds and sets a predefined value for the wheel slip. The brake actuation signals are generated either by activation operations of the brake pedal by the driver or by a control system for controlling the vehicle dynamics or driving stability.
Known anti-lock brake systems (for example EP 0 644 836 B1) control the brake pressure of the individual wheels of a motor vehicle in such a way that the wheels are prevented from locking during braking and the steering capability of the motor vehicle is thus maintained by adapting the wheel brake pressure to the respective adhesion of the tire to the road pavement. This adaptation is carried out in that, when unacceptably high slip occurs, the brake pressure acting on the individual wheels is decreased intermittentlyxe2x80x94and thus discontinuouslyxe2x80x94and increased again using brake pressure control valves.
A more recent development is electromotively activated brake systemsxe2x80x94referred to as EMB systemsxe2x80x94which improve the safety and the comfort of the operation of motor vehicles and which are suitable in particular for use in systems for controlling the dynamics and stability of vehicles, traction controls, xe2x80x9cintelligentxe2x80x9d travel speed controllers etc. (see for example DE 198 13 194 C2). Such brake systems also greatly reduce or entirely eliminate the considerable expenditure on master brake cylinders, brake pressure lines etc. of conventional brake systems.
A method is known for controlling the slip of a wheel of a vehicle which determines a slip actual value and a first manipulated variable in accordance with a comparison between the slip actual value and a slip setpoint value in order to apply a brake pressure to the wheel, the first manipulated variable also being determined in accordance with the timing characteristics of the speed of one or more wheels of the vehicle (DE 199 26 672 A1). In order to condition an actuation variable for actuating a valve, the method determines a minimum actuation variable, compares an actuation variable determined by a controller with the minimum actuation variable and adjusts the actuation variable using earlier and/or later actuation variables to form a new actuation variable if the actuation variable is smaller than the minimum actuation variable.
An anti-lock brake system for a vehicle wheel is also known in which the wheel speed and the brake pressure are determined and slip actual values are formed using a reference speed and are used to control the brake pressure at the wheel brake (DE 40 34 814 A1). Here, the adjustment phases and brake pressure control phases alternate with one another, and during the adjustment phases the coefficient of friction is continuously determined from the brake pressure and the deceleration of the wheel. In these phases, in each case the maximum value of the coefficient of friction and in addition the slip value are determined when the maximum coefficient of friction is reached. In each case a somewhat smaller setpoint slip value is formed from the slip value and in the following adjustment phase it is fed to a slip controller as a setpoint slip value. When the wheel slip grows, the system is switched over during the adjustment phases beyond a predefined value to a brake pressure control mode during which a reduced brake pressure is set. After the slip drops below the value, an increased brake pressure is set and the slip controller is activated with the new setpoint slip if the slip is smaller than a limiting value.
A system which controls vehicle dynamics which is also known has a subordinate vehicle dynamics computer downstream of which a brake controller is connected (DE 43 05 155 A1). The latter has a channel for adjusting the yaw rate to a setpoint value and a channel for limiting the floating angle. The computer predefines the setpoint value and the limiting value and determines soft channel is effective on the basis of the driving situation. Both channels generate an adjustment signal which is conducted via a control amplifier with proportional and derivative component. An activation signal which is used to control the brake pressure at the wheel brakes is formed from the signal components which are produced, the instantaneous driving situation being categorized in one of a plurality of classes in both cases and this categorization also being used to define the wheels at which the brake pressure is controlled.
In particular within the framework of a vehicle dynamics control system it is desired to adjust a wheel to a predefined longitudinal slip value. The latter can of course also lie beyond the adhesion maximum of the tire, that is to say in the unstable region of the static friction curve or xcexc curve, in order thus to significantly reduce the lateral guidance of the wheel. Conventional PID controllers which are used in known ABS systems can cause difficulties in an electromotively activated brakes, particularly beyond the adhesion maximum.
The invention is based on the object of providing a brake control method and a brake system which permits satisfactory control of motor vehicle wheels to predefined longitudinal slip values, such as are desirable for example in a vehicle dynamics control system, even in EMB systems. This object can be, for example, achieved according to the invention by a method for controlling the braking process in a motor vehicle, comprising the steps of:
converting brake actuation signals into brake application forces,
transmitting the wheel speeds of at least some of the wheels to a control system,
calculating a wheel slip by the control system from the wheel speeds, and setting a predefined value for the wheel slip,
setting the predefined value for the wheel slip by a proportional control and a setpoint brake pressure which is defined in parallel therewith.
Another method for controlling the braking process in a motor vehicle according to the present invention, comprises the steps of:
brake actuation signals are converted into brake application forces,
the wheel speeds of at least some of the wheels are transmitted to a control system,
a wheel slip is calculated by the control system from the wheel speeds, and a predefined value is set for the wheel slip,
the predefined value for the wheel slip is set by a proportional control and a setpoint brake pressure which is defined in parallel therewith, and
by setting the control to a longitudinal slip above the maximum of the coefficient of static friction, the lateral guiding force of the wheel is reduced, and the driving behavior of the motor vehicle is thus stabilized.
The predefined value for the wheel slip can be set by means of a proportional plus derivative control. The predefined value for the wheel slip can also be set by means of a proportional integral control with a very small integral component. The setpoint brake pressure can be determined by means of the wheel torque which is transmitted from the wheel to the underlying surface instantaneously. The wheel torque which can be transmitted from the wheel to the underlying surface instantaneously is determined from the torque balance at the wheel. The wheel torque which is transmitted from the wheel to the underlying surface instantaneously can be determined according to the equation Minst.=xe2x88x92Mbrake+{dot over (xcfx89)}xc2x7"THgr", wherein Mbrake is the brake torque caused by the brake, {dot over (xcfx89)} is the wheel acceleration values and "THgr" is the wheel inertia values, and in that the defined setpoint brake pressure with which intervention into the brake control system is carried out is calculated using a wheel brake pressure which corresponds to this transmitted wheel torque.
An embodiment according to the present invention can be a brake system with control of the braking process in a motor vehicle, comprising:
wheel speed sensors which sense the speeds of the wheels of the motor vehicle and transmit them to a control system,
a control system which evaluates the sensor signals, calculates slip values of the individual wheels from the wheel speeds and generates control signals for the wheel brakes, and which contains a controller which is embodied as a proportional controller and which controls a predefined value for the wheel slip,
a setpoint brake pressure being determined by the control system and said setpoint brake pressure being used, in addition to the adjustment, to set the brake pressure at the wheel brakes, and
having brake actuators which convert the control signals into activation operations of the wheel brakes, wherein
the control system sets the control to a longitudinal slip lying above the maximum of the coefficient of static friction and as a result reduces the lateral guiding force of the wheel and stabilizes the driving behavior of the motor vehicle.
The setpoint brake pressure can be determined by the control system by means of the wheel torque which is transmitted from the respective wheel to the underlying surface instantaneously The wheel torque which is transmitted from the respective wheel to the underlying surface instantaneously can be determined by the control system from the torque balance at the wheel. The predefined value for the wheel slip is set by a proportional controller and a setpoint brake pressure which is defined in parallel thereto (additionally).
The advantages of the invention are in particular the low level of expenditure which is necessary for its practical application. In addition, it has become apparent that even anti-lock brake systems can be significantly improved using the invention.